1. Field of the Invention
The present invention relates to a display apparatus displaying a three-dimensional image and a display method of displaying a three-dimensional image, and in particular, to a display apparatus displaying a three-dimensional image which apparatus comprises an optical plate, i.e., a ray control element controlling a direction in which light rays are applied, as well as a display method of displaying a three-dimensional image.
2. Description of the Related Art
An integral imaging method (also simply referred to as an II system below) for displaying a large number of parallax images is known as a system for recording a three-dimensional image and reproducing it as a three-dimensional image. The II system is also referred to as an integral photography method (IP method) and is a method of displaying three-dimensional image which method belongs to a method of ray reproduction.
In general, when an observer views an object with both eyes and if θ1 denotes the angle between a near point at a short distance and both eyes and θ2 denotes the angle between a far point at a long distance and both eyes, the angles θ1 and θ2 vary depending on the positional relationship between the object and the observer. The difference in angle (θ1-θ2) is called a binocular parallax. Human beings can react sensitively to a binocular parallax to three-dimensionally view the object.
In recent years, efforts have been made to develop a display apparatus (three-dimensional display) that displays a three-dimensional image without the need for glasses. Many such three-dimensional apparatuses use a two-dimensional display module (what is called a planar display) that displays a normal two-dimensional (2D) image. A ray control element is placed in front of or behind the display module to control certain light rays. Such a three-dimensional image display apparatus displays a three-dimensional image by utilizing the above binocular parallax to control the angle of light rays so that when the observer views the display apparatus, the light rays appear to be emitted by an object at a distance of about several centimeters from the display module. Three-dimensional images can now be displayed as described above because the display module has an increased definition, so that an image of a somewhat high definition can be obtained even if light rays from the display module is divided into pieces corresponding to several angles (called parallaxes).
FIG. 1 schematically shows the configuration of a three-dimensional display apparatus for the II system which provides parallaxes in a horizontal direction, as an example of a three-dimensional image display apparatus of no glass type.
FIG. 1 schematically shows the arrangement, in a horizontal plane, of a display module surface 61 on which a two-dimensional image is displayed and an observer 64 and a ray control element 68 that controls light rays 67 from a display module surface 61. In the display apparatus shown in FIG. 1, many pieces of image information are displayed on the two-dimensional display module 61, which displays a two-dimensional image. The observer 64 observes an image via an array plate provided in front of the display surface and corresponding to a ray control element having optical apertures and blocking sections 63. A three-dimensional image is thus displayed depending on an observing direction. The optical apertures 62 correspond to slits, pinholes, micro-lenses, lenticular lenses, or the like. In the specification, the optical apertures may be simply referred to as apertures because they can act optically as apertures even without actual apertures.
With this display apparatus, as shown in FIG. 1, the observer 64 at a position a located at a distance L from the array plate 63 can view an image with a parallax number 3. The observer 64 at a position β can view an image with a parallax number 2. Similarly, the observer 64 at a position γ can view an image with a parallax number 1. The II system enables multi-parallax display so that in spite of his or her motion, the observer 64 can view an image corresponding to his or her position. Since motion parallaxes can thus be displayed, the II system enables natural three-dimensional viewing. Further, with the II system, light rays reproducing a three-dimensional image follow paths similar to those used if an object (subject) is actually provided. Consequently, this method is also excellent in that binocular rivalry does not occur.
Methods for creating and displaying a parallax image via the apertures as pixel information are roughly classified into two types, a method of mapping images by causing pixels to generate light rays that reproduce a three-dimensional image and a method of mapping images by reversely following light rays from the viewpoint position of the observer toward the pixels. In the specification, these two methods are distinguished from each other by referring to the image mapping using the former method as the II system and referring the latter as a multi-eye display method also referred to as a multi-parallax stereoscope or parallax barrier method or the like.
With the II system, unlike the parallax barrier method or the like, a pencil of light rays are not controlled so as to be directed to the position of the observer's eye but so that a number of light rays corresponding to the number of parallaxes are emitted to the observer via all the apertures at almost equal intervals. Accordingly, the II system is excellent in motion parallaxes obtained if the observer moves. However, if the viewpoint position is fixed, the II system provides a smaller number of constituent pixels contributing to generation of a three-dimensional image at a certain angle than intrinsically two-dimensional (2D) display modules. The II system thus provides a lower resolution than three-dimensional image display apparatuses (multi-eye display apparatuses) that emit light rays to the position of the observer's eye.
In general, a certain specified resolution is required for character display or for example, spherical display having components oblique to lenses or slits. However, it is difficult for an orthographic-projection three-dimensional display apparatus (II system) comprising lenticular lenses or slits to provide fine character expressions or smooth curve displays owing to a limit to the resolution, determined by the pitch of the lenticular lenses or slits. If a three-dimensional image having a depth is to be displayed, then disadvantageously, anti-aliasing may make the boundary between a background and an image notched, or in a near-side region (a space in front of the apparatus and close to the observer), the observer may see a double image near an near-side limit of 3D image position. This is because crosstalk causes the observer to see not only a parallax image to be originally viewed but also an adjacent parallax image, thus preventing the observer from seeing only the correct parallax image.
H. Hoshino, F. Okano, H. Isono and I. Yuyama, “Analysis of resolution limitation of integral photography”, J. Opt. Soc. Am, A15 (1998) 2059-2065 discloses a three-dimensional display apparatus that sets the gap between the surface of each lens and a two-dimensional pattern display module equal to the focal distance of the lens. With this three-dimensional display apparatus, parallax rays toward the observer are formed into an image, via the lenses, on the display module displaying a two-dimensional pattern. Under these conditions, the parallax rays are formed into one pixel to reduce crosstalk.
Jpn. Pat. Appln. KOKAI Publication No. 07-287195 discloses a glasses-less three-dimensional display apparatus using lenticular lenses intended to improve image quality. This image display apparatus is configured to sufficiently separate adjacent images from each other. Specifically, with this three-dimensional display apparatus, compressed images formed on an image sheet are observed via a lenticular screen consisting of a plurality of the lenticular lenses. With this three-dimensional display apparatus, each compressed image is formed on the image sheet in association with one lenticular lens. A band-like buffer area is provided between the adjacent compressed images. With this apparatus, when the adjacent compressed images are observed from a continuously varying viewpoint position via the lenticular screen, two images corresponding to these compressed images appear to be separated from each other by the buffer area. Consequently, the display apparatus reduces crosstalk. This configuration is adopted because with three-dimensional display apparatuses, visible light rays for adjacent parallax images may hinder three-dimensional viewing.
Japanese Patent No. 2874985 discloses a three-dimensional video apparatus in which a lenticular lens plate is placed opposite an image display surface to allow a three-dimensional video to be viewed from multiple directions. In this three-dimensional video apparatus, the image display surface is placed within the focal distance of the lenticular lens plate to diffuse light rays transmitted through the lenticular lens plate. The pixels on the image display surface are aligned with the lenticular lenses so that even a dark part between the pixels does not result in a dark part in a viewing area. With this display apparatus, to eliminate moiré, which causes the dark parts to appear to be striped, gap length is set equal to or smaller than the focal distance of the lenses.
Jpn. Pat. Appln. KOKAI Publication No. 2001-275134 discloses a three-dimensional display apparatus for an integral photography system which supplies the observer's eye with a three-dimensional image at the optimum position. The three-dimensional image display apparatus includes a passive array having apertures and a second array that displays an image to be displayed. The display array includes a set of sub-arrays associated with the corresponding apertures in the passive array. Each point in each sub-array is associated with the corresponding aperture in the passive array and includes information on a position on a three-dimensional image to be displayed. Light rays traveling from the points in the sub-arrays to the associated points in the passive array converge at the corresponding points on the three-dimensional image.
This display apparatus comprises control means for controlling the directions of light rays and thus the position of the three-dimensional image with respect to the passive and second arrays. The control means is suitably provided so as to control the distance between the passive array and the second array. Japanese Patent No. 2874985 enables the gap and thus the display position to be varied to provide a three-dimensional display that is optimum for the observer. This patent does not refer to light rays for adjacent parallax images.
A problem with the II system is that when a three-dimensional image is reproduced at a position away from the display surface, a pencil of light rays assigned via the apertures or lenses spread to rapidly reduce the resolution. The disadvantageous decrease in resolution will be described below with reference to FIGS. 2, 3, and 4.
A cycle per radian (cpr) β is used as a measure for expressing the resolution of a three-dimensional display apparatus. The cycle per radian β is an index indicating the number of cycles in which the brightness of light rays per radian can be displayed. As shown in Jpn. Pat. Appln. KOKAI Publication No. 07-287195, if with the II system, a three-dimensional image 73 is displayed near the display apparatus (the distance zi from the observer 64 to the three-dimensional image 73 is also sufficiently short) as shown in FIG. 2, the resolution is determined by a pixel pitch called a Nyquist frequency (βnyq) and viewed by the observer 64 via the lenses or the apertures 62. The pitch of the apertures 62 and the distance between the observer 64 and the apertures 62 or lenses are defined as pe and L, respectively. Then, the resolution (βnyq) limited by the aperture pitch pe is expressed by:βnyq=L/2pe   (1-1)
Then, if the object image 73 is reproduced at a position away from the display surface of the display module 1 (the distance zi from the observer 64 to the three-dimensional image 73 is also sufficiently long) as shown in FIG. 3, a pencil of light rays assigned via the apertures or lenses spread to rapidly reduce the resolution. If the object image 73 is to be reproduced in the front- or far-side region, when the maximum value of the resolution calculated for the group of light rays 67 emitted through one of the slits 68 in order to reproduce the three-dimensional image 73 is defined as αimax, a resolution βimax determined by the spatial frequency of the object as viewed by the observer is expressed by:βimax=αimax Zi/|L-Zi|  (1-2)
An actual resolution Bimax is the lower of (1-1) and (1-2) and is thus expressed by:βimax=min(βimax, βnyq)  (3)
In FIGS. 2 and 3, a coordinate Z corresponds to a distance extending from the observer 64 along a normal to the display surface 1 of the display module 1. The distance from the apertures or lenses to the object image 73 is denoted by (L-Zi). The distance between the display surface 61 and the observer 64 is denoted by Z(=L+g). The maximum value of the resolution αimax depends on the angle αcpr formed by straight lines joining one of the slits 68 and the horizontal ends of the object image together. The near-side region means a front area with respect to the observer 64 which corresponds to an observer side of the display apparatus. The far-side region means a rear area with respect to the observer 64 which corresponds to a side of the display apparatus which is opposite the observer. The three-dimensional image display apparatus for the II system can form a three-dimensional image in the near- or far-side region.
Expression (1-1) indicates that the resolution of the three-dimensional image increases with decreasing aperture pitch, that is, increasing definition of the display surface. However, a reduction in the pixel pitch of the display surface of the display module may for example, require processes executed by the display module to be changed. Accordingly, this cannot be easily realized.
Further, as is apparent from Jpn. Pat. Appln. KOKAI Publication No. 07-287195, if the three-dimensional image is formed near the display surface, the resolution βnyq, limited by the aperture pitch, is lower than the resolution βimax, determined by the spatial frequency of the object. Consequently, the resolution βnyq, limited by the aperture pitch, is predominant. On the other hand, the farther the three-dimensional image is from the display surface, the shorter the distance Zi from the observer in Expression (1-2) is. Consequently, the resolution βimax becomes predominant. For example, the resolutions determined from Expressions (1) and (1-2) for a certain number of parallaxes and a certain viewing area angle has the relationship shown in FIG. 4. In FIG. 4, an axis z indicates the distance from the observer to the three-dimensionally displayed object, and z=L(m) denotes the position of the display apparatus. The axis of ordinate indicates the resolution βnyq, determined by Expression (1-1) and depending on the lens pitch, and the resolution βimax, determined by Expression (1-2) and by the density of light rays emitted from one of the lenticular lenses. FIG. 4 indicates that if the object is displayed near the display surface, that is, in an area with a near-side amount zn=zno(m) and a far-side amount zf=zfo(m), βnyq (1-1), determined by the lens pitch, is lower than βimax and is thus predominant. If the object is displayed in an area with a near-side amount larger than zn or a far-side amount larger than zf, βimax (1-2), determined by the density of light rays from the apertures, is predominant.
Crosstalk is a phenomenon in which the observer views light rays from a parallax image that is intrinsically not to be seen. Specifically, the defocusing of the lenses, a diffusion plate, or the like may prevent a pencil of light rays seen by the observer from being focused on the two-dimensional pattern display surface. The observer thus views light rays containing adjacent parallax images. The crosstalk may make curves and oblique lines notched, resulting in a degraded image in which the object to be displayed is mixed with its background. If for example, a round ball is to be displayed in a far-side direction, the contour of the round ball may disadvantageously be notched.
The crosstalk phenomenon occurs more markedly in parallax images to be emitted by two-dimensional display devices in certain ray directions, the observer views light rays for not only the desired parallax images but also the adjacent ones. The crosstalk also results from the defocusing of the lenses, the insertion of a diffusion film, or the like. The crosstalk may reduce the resolution.